Process for producing highly pure isoprene

ABSTRACT

Highly pure isoprene is separated from a C5 hydrocarbon fraction divided from cracking of petroleum, particularly naphtha, containing C5 paraffin, C5 monoolefines, C5 diolefines, C5 acetylenes, a small amount of C5 and C6 hydrocarbons and sulfur compounds by removing a major portion of cyclopentadiene and acetylenes contained in said fraction as polymerization retarding impurities according to dimerization by heat soaking and distillation, and distillation, respectively, and treating the fraction with sodium containing aliphatic monohydric alcohol having one to 18 carbon atoms in amount of not more than that of sodium by equivalent to remove the remaining cyclopentadiene, acetylenes and sulfur compounds.

CYCLICAL PROCESS FOR THE DEHYDROGENATION OF SATURATED HYDROCARBONS The production of olefins and diolefins from paraffins of the same carbon structure is based essentially on simple catalytic dehydrogenation processes. This type of conversion involves a balanced reaction conforming to the laws of thermodynamics; in order to achieve industrially acceptable performances it is necessary to use rather unfavorable operating conditions: high temperature, lower hydrocarbon pressure.

These conditions in most cases result in a rapid deactivation of the catalyst and accordingly in the requirement of numerous regeneration steps which reduce substantially the life time of these catalysts.

In view of avoiding these numerous drawbacks, derived from the presence of a thermodynamic equilibrium, methods for selective removal of the hydrogen formed during the dehydrogenation, have been contemplated in order to displace the equilibrium towards a larger production of dehydrogenated compounds.

The most economical way therefor consists of introducing air or oxygen for converting the hydrogen to water by combustion. A number of processes has been proposed in the case of preparing diolefins by controlled oxidation of olefins in the presence of catalysts, but the rare examples of production of olefins and diolefins by oxidizing paraffms are generally characterized by poor performances, mainly as the result of the degradation reactions involving a complete combustion due to the simultaneous presence, in the gaseous reaction medium of hydrocarbons and oxygen.

It is an object of the present invention to provide a process for manufacturing olefins and diolefins from paraffin, said process being characterized by a first stage of contacting said paraffins with a metal compound or compounds mixture containing molybdenum at least in major part in the form of molybdate, said molybdate being selected from the'group consisting of iron, cobalt and nickel molybdates, followed with a second stage where the molybdate at least partially reduced to the molybdite form is reoxidized, before being again contacted with a new paraffin charge, at least partially to the molybdate form by contacting the same with an oxygen-containing gas, the oxidation of the paraffins being conducted in the absence of free oxygen and the oxidation of the molybdite being conducted in the absence of paraffins, said two stages being optionally carried out in the reverse order.

This process offers, with respect to the known processes for oxidation of paraffins, numerous advantages, mainly a high selectivity and a high yield even at a moderate reaction temperature and relatively high pressure, Moreover, due to the operation in the absence of oxygen it is possible to avoid secondary reactions of degradation which otherwise occur when hydrocarbons and oxygen are present in the reaction medium, said reactions resulting in the formation of the carbon dioxide in non-negligible amounts thereby leading on the one hand to the loss of a portion of the paraffins and on the other hand to temperature increases which are detrimental to the oxidation reaction.

The increase in selectivity is of particular interest in the case of branched hydrocarbon chains since the process of the invention avoids the breaking of the lateral chains.

The risks of explosions which might occur irrespective of the mean ratio oxygen/paraffin, are also avoided.

Moreover, according to the present process a small amount of molybdate as such or incorporated to such carriers as alumina, silica and the like, provides for the conversion of an almost unlimited amount of paraffins with excellent performances.

The paraffins which may be used according to the present invention are linear or branched paraffins containing from two to 10 and preferably from four to eight carbon atoms.

The process may be carried out by means of any apparatus whereby is achieved an alternate contact of the molybdate with the gaseous phase containing the paraffms, either alone or diluted with an inert gas such as nitrogen, carbon dioxide or steam, and thereafter of the reduced molybdate with the oxygen-containing gaseous phase, the process being by no way limited to the use of any particular apparatus.

It is obvious that the order of the two stages can be reversed, provided that there is fulfilled the condition according to which the molybdenum is initially, at least in major part (preferably in totality), in the form of molybdate when in the presence of paraffins and at least partly in the form of molybdite when in the presence of oxygen.

For instance the reaction can be carried out in a reaction vessel operated under dynamic conditions; reactants, injected under a pressure between 0.1 and 2 atmospheres, are successively introduced into the reac tion vessel containing the slid mass in the form of grains, extrudates or powder, having a size elected, in accordance with the type of operation: in a fixed bed for example there can be used particles of a diameter for instance between 0.1 and 50 mm; in a fluid bed, smaller diameters will be preferred in order to obtain a good stability of the bed, for instance diameters between 0.01 and 0.2 mm; in a moving bed, i.e. when the solid catalyst is circulated in the reaction vessel, intermediate sizes of, f.i.,'0.05 to 0.5 mm can be used.

All of these values are only given for illustrative pur poses since the sizes are to be selected according to each particular case of use, with consideration for the type of solid used, its mechanical properties, the operating conditions and the feature of the installation.

The successive introduction of the reactants, paraffin and oxygen or air, may be regulated by a system of automatically operated valves wherein each valve can be open only after the closure of the other valves.

In order to improve the safety of operation and avoid any risk of casual mixing of paraffin and oxygen, provision can be made for an intermediate supplemental injection of an inert gas (nitrogen, steam, carbon dioxide, for example).

The operating cycle will thus include the successive steps of l. Injecting paraffms on the molybdate, resulting in the production of olefins and diolefins and in at least a partial reduction of the molybdate to molybdite.

2. blowing off with an inert gas 3. injecting oxygen on the molybdite and oxidation of the molybdite to molybdate 4. blowing off with an inert gas 5. new cycle such as l PROCESS FOR PRODUCING HIGHLY PURE ISOPRENE This invention relates to an improvement in process for purifying isoprene, and more particularly to an improvement in process for preparing polymerization grade isoprene from the C, hydrocarbon faction derived from the cracking of petroleum, especially naphtha.

The C, hydrocarbon fractionof naphtha-cracked oil usually contains several ten kinds of C C, hydrocarbon components, and the boiling points and relative volatilities to isoprene of the main components are shown in Table 1.

As is apparent from Table 1, many components very similar to isoprene in boiling point and relative volatility are contained in the C, hydrocarbon fraction, and therefore it is quite impossible to separate and purify the isoprene at a high purity according to the ordinary distillation process. Particularly among the impurities contained in the C, hydrocarbon fraction, the components, for example, cyclopentadiene, acetylenes, allenes and sulfur compounds that have a poisoning effect upon the so-called Ziegler-type catalyst and retard a smooth progress of polymerization reaction, must be removed to such a degree that the polymerization is not substantially affected.

l-leretofore, various removal and purification methods have been applied in a combination with ordinary distillation process alone or together with, for example, extractive distillation process, azeotropic distillation. process, absorption process, organic metal or metallic sodium treatment process, molecular sieve process, etc. However, these processes have advantages as well as disadvantages at the same time in respect to the economy, safety, isoprene yield, product purity, etc.

dicyc::opentadiene The present inventors have made studies on a process for removing said polymerization-retarding impurities and recovering highly pure isoprene with a high efficiency from the C, hydrocarbon fraction to overcome said disadvantages heretofore encountered, and have found that highly pure isoprene can be obtained with a high efficiency by the following processes:

namely, a major portion of the polymerization-retarding materials such as cyclopentadiene, acetylenes and allenes contained in the C, hydrocarbon fraction are preliminarily removed by prior well-known process and, thereafter, remaining polymerization-retarding materials such as sulfur compounds, a-acetylenes, and cyclopentadiene are removed by treating with a mixture of metallic sodium and a minor amount of aliphatic alcohol without almost any loss of isoprene;

The process for purifying olefines through contact with sodium'to remove a very small amount of impurities contained in the olefines is well known, and has been generally used on a laboratory scale. When there are contained a large amount of readily reactive impurities such as water and sulfur compounds, sodium is poisoned in case of said well-known sodium treatment process alone, so the a-acetylenes, allenes and cyclopentadiene remain as'they are. Therefore, in the case of sodium treatment, is necessary-to provide a step for removing water and sulfurin advance. As the sulfur compounds contained in the C, hydrocarbon fraction, there are mentioned hydrogen sulfide, mercaptanes, sulfides and carbon disulfide. Carbon disulfide cannot be removed by alkali washing, and thus it is necessary to remove it by some other means.

An object of the present invention is to provide a process for removing polymerization-retarding substances more effectively.

That is to say, the present invention is to provide a process for separating and purifying isoprene from a C, hydrocarbon fraction containing C, paraffins, C, monoolefines, C, diolefines, C, acetylenes, a small amount of C and C, hydrocarbon fractions, and sulfur compounds which comprises a. removing a major portion of cyclopentadiene and acetylenes contained in the fraction by previously known procedures, and

b. treating the fraction with sodium, preferably in a form of dispersion, containing small amount of aliphatic monohydric alcohol having one to 18 carbon atoms to remove the remaining polymerization-retard ing materials such as cyclopentadiene, acetylenes and sulfur compounds.

As processes for removing cyclopentadiene, it is well I known that C, hydrocarbon fraction is heated to dimerize cyclopentadiene and the resulting dimer (dicyclopentadiene) is removed by distillation. These processes are disclosed, for example, in U.S. Pat. Nos. 2,768,224 and 2,971,036.

As a process for removing acetylenes, it is well known that acetylenes are removed by azeotropic distillation with isopentane contained in C, hydrocarbon fraction, as disclosed in U.S. Pat. No. 2,851,505.

As a process for separating diolefines from paraffines and monoolefines having boiling points close to that of diolefines, it is known that extractive distillation process is available.

In the present invention it is preferable that major portion of cyclopentadiene is removed by above-mentioned dimerization process and major portion of acetylenes are removed by straight distillation in the presence of isopentane contained in the C, hydrocarbon fraction before the present sodium-alcohol treatment.

Extractive distillation process may be applied after the sodium-alcohol treatment, but preferably applied before the treatment from the viewpoint of efficiency of the treatment. As extractive solvent used in the extractive distillation process, such conventional solvents, as acetonitrile, dimethylformamide, acetone, furfural and N-methyl pyrrolidone, etc., may be used.

In the process of removing a major portion of cyclopentadiene the C hydrocarbon fraction is led to a dimerization reactor and heated to 90 130C for l 30 hours, preferably 1 hours, and the resulting dimer of cyclopentadiene is removed together with other high boiling point materials in successive distillation column. In this process cyclopentadiene is removed to about 3 percent or less by weight.

In the process of removing a major portion of acetylenes, the acetylenes contained in the C hydrocarbon fraction form an azeotropic mixture with isopentane also contained in the fraction and are removed to about 10 200 ppm by distillation, though it depends on conditions of the distillation.

Allenes especially 1,2-butadiene, are removed in this process to such a degree that they have substantially no effect on polymerization.

A process for obtaining highly pure isoprene by removing substantially all of cyclopentadiene-and acetylenes according to the procedures of (1) heat soaking (dimerization of cyclopentadiene), (2) removal of high boiling point materials, (3) removal of low boiling point materials and (4) extractive distillation is disclosed, 'for example, in British Patent Specification No. 1,137,268. However, it is necessary in said process to make the operating conditions of said steps l) and (2) severe to remove substantially all of cyclopentadiene and acetylenes, and therefore the process is not always satisfactory in economy and yield of isoprene. Furthermore, the removal of sulfur compounds is a problem in the process.

In the present invention, said disadvantages have been overcome, and these impurities are efficiently removed, whereby highly pure isoprene can be obtained economically.

According to the conventional process, the extract derived from the extractive distillation column is introduced to the second extractive distillation column to remove diolefines and acetylene having larger solubilities than that of isoprene. Then, to remove the polymerization-retarding impurities to ppm order, it is necessary to provide another distillation columns, topping and tailing columns for removing low boiling point and high boiling point components respectively. On the other hand, in the present invention a major portion of acetylenes are azeotropically distilled off with isopentane contained in the C hydrocarbon fraction in the topping column, and then the remaining acetylenes, cyclopentadiene and sulfur compounds are readily removed by the sodium treatment in the presence of aliphatic alcohol in amount of not more than that of sodium by equivalent.

In a conventional sodium treatment process, when a considerable amount of said impurities are involved,

sodium is often poisoned and consequently an excess amount of sodium must be used. In that case, however, there has been the possibility that the excess sodium causes polymerization of isoprene itself. However, the

present inventors have found that the amount of sodi-- um can be considerably reduced when aliphatic alcohol having one to 18 carbon atoms is used together with sodium in amount of not more than that of sodium by equivalent, and further the polymerization of isoprene can be effectively prevented thereby. Furthermore, the present inventors have found that the sodium-alcohol treatment can be sufficiently applied, even if a considerably large amount of polymerization-retarding impurities are present.

In the present invention, the acetylenes remaining as the column bottom distillate in the topping column and cyclopentadiene and sulfur compounds remaining in the tailing column are removed by the sodium-alcohol treatment. Sodium is usually dispersed in a medium as particles having sizes of l to microns and used "in such a dispersed state. The sodium dispersion can be prepared according to the conventional, well-known procedure.

As the medium for the sodium dispersion, media that are inactive to sodium and have a boiling point higher than the melting point of sodium, for example, toluene, xylene, a mineral oil, petrolatum, naphthalene, tetraline, heptane, etc. are usually used.

The dispersion containing 20 60 percent by weight, preferably 40 50 percent by weight of sodium, is used in the sodium-alcohol treatment.

The alcohols used in the sodium-alcohol treatment are aliphatic monohydric alcohols having one to 18 carbon atoms, for example, as methyl, ethyl, propyl, butyl, amyl, hexyl, octyl, nomyl, decyl, lauryl, palmityl and stearyl alcohols.

lsopropyl, tert-butyl, isoamyl, octyl and lauryl alcohols are preferably used.

According to the present invention, 0.5 to 1.5 parts, preferably 1.05 to 1.2 parts by equivalent of sodium is used for one part by equivalent of polymerization-retarding impurities, that is, cyclopentadiene, acetylenes and sulfur compounds. The amount of said alcohols used is not more than that of sodium by equivalent and l 10 times, preferably 2 5 times that of the sulfur compounds by equivalent.

It is desirable to use the alcohol after a drying treatment to reduce the amount of sodium to be used. The alcohol can be used directly or after diluted with a solvent inactive to sodium.

Any temperature can be applied to the sodium-alcohol treatment so long as the isoprene is not polymerized and consequently consumed at that temperature, but usually a temperature below 40C is economical and preferable for carrying out the treatment.

' The reaction pressure of the reaction system of the sodium-alcohol treatment is usually atmospheric, because the handling is advantageous under the atmospheric pressure. However, the treatment can be carried out under pressure.

Now, the present invention will be explained in detail, referring to examples and drawing, but will not be limited to the examples and can be modified within the scope and spirit of the present invention.

The accompanying drawing shows a flow diagram of one embodiment of the present invention, referring to Example 1.

EXAMPLE 1 Naphtha-cracked C hydrocarbon fraction having a composition (percent by weight) as shown in Table 2 was used as a feed stream 1.

The feed stream 1 was charged into a dimerizer A and most of cyclopentadiene in the feed stream was dimerized to dicyclopentadiene at a soaking temperature of 110 120C for a retention time of 6 hours,

and the cyclopentadiene content of the treated fraction was reduced to 1.00 percent by weight.

The process stream 2 leaving the dimerizer A was led to the first distillation column 13 having 120 plates for removing the high boiling point components, and was operated with a reflux ratio of 5.0 at an overhead temperature of 63C and a bottom temperature of 95C under a pressure of 2.7 Kglcm Thus, most of hydrocarbons having boiling points higher than that of isoprene were removed, and hydrocarbon stream 4 containing 16.6 percent by weight of isoprene and 0.21 percent by weight of cyclopentadiene was obtained. Recovering rate of isoprene was 95 percent.

The distillate stream 4 of the column B was led to the first topping column C having 120 plates for removing the low boiling point components, and was operated with a reflux ratio of 5.0 at an overhead temperature of 62C and bottom temperature of 95.5C under a pressure of 2.7 Kg/cm absolute, whereby the bottoms stream 5 containing 24.2 percent by weight of isoprene was obtained.

In this procedure, 2-butyne and Z-methyl-l-butene- 3-yne were almost all eliminated from the process stream as the overhead distillate, and both Z-butyne and Z-methyl-l-buten'e-S-yne in the process stream were reduced to less than 100 ppm.

The bottoms fraction 5 of the column C was then led to the extractive distillation column D having 100 plates, and was operated with a reflux ratio of 3.0 at an by weight of water was selected and fed to the column at 10th plate from the top of column in a proportion of the solvent to the fed C hydrocarbon fraction of 6 l by weight.

The bottoms stream? of the column D containing diolefines such as isoprene and acetylene was fed to the stripping column E together with the solvent, where the process stream was separated from the solvent, acetonitrile.

The overhead fraction 10 was then fed to the water washing column F, where the acetonitrile still contained in the isoprene stream was eliminated to less than 5 ppm, and then the isoprene stream 11 was fed to the second topping column G having 120 plates for removing the low boiling point components, and was operated with a reflux ratio of 280 at an overhead temperature of 56C and bottom temperature of C under a pressure of 2.0 Kg/cm absolute. in the column G, the components having lower boiling points than that of isoprene, such as 1,4-pentadiene, 2-butyne, etc. were eliminated from the isoprene stream to such a degree that they will no more retard the polymerization, and at the same time water was eliminated therefrom to a trace into the stream 12.

The bottom stream 13 of the column G having the composition shown in Table 3, was allowed to come in contact with the treating agents, sodium and alcohol fed through 14 and 15 in the reactor H. As the treating agent, a sodium dispersion having sizes of 10 to microns and concentration of about 40 percent by weight in xylene was used. At that time, isopropyl alcohol was used as the alcohol component.

The amounts of the treating agents were as follows,

Na/polymerization-retarding impurities (cyclopentadiene, acetylenes and sulfur compounds) l.10/l (by equivalent) isopropyl alcohol/sulfur compounds I equivalent) The polymerization-retarding impurities, that is acetylenes, cyclopentadienes and sulfur compounds, were eliminated therefrom to a trace through reaction at a reaction temperature of 30C for a reaction time of 3 hours. In this procedure, the loss of isoprene was negligibly small.

The stream 16, whose composition was shown in Table 3, leaving the reactor H was led to the centrifuge 1, whereby the reaction product and the unreacted metallic sodium were all separated from the isoprene stream and withdrawn from the line 18. The effluent 18 was fed to the second distillation column J for remov- Composition by weight) Stream l3 Stream l6 lsoprene 94.3 94.1 trans-piperylene 3.15 3.18 cyclopentadiene 0.85 (less than 10 ppm) Z-Pentene 0.63 0.63 Z-Methyl-Z-butene 0.47 0.48 Cyclopentene 0.24 0.24 Cyclopentane 0.19 0.20

l-Pentyne 0.12 (less than 10 ppm) tl-l-lzutss -vn 0- (less than 10 ppm) 1,4-Pentadiene 0.01 0.01 Sulfur compounds (as S) (50 ppm) (less than 10 ppm) Xylene (medium of Na dispersion) 1.16

TABLE 4 Component Composition isoprene 99.8 or more monoolefines 1,000 ppm or less 1,4-pentadiene 100 ppm or less piperylene 50 ppm or less cyclopentadiene 5 ppm or less acetylenes ppm or less sulfur compounds (as S) 5 ppm or less EXAMPLES 2 5 By treating C hydrocarbon fractions obtained by naphtha cracking, containing 12.83 percent by weight of isoprene as shown in Table 5 according to the same manner as in Example 1, the fractions as shown in Table 6 were obtained as the feed for the sodium-alcohol treatment. Said fractions were divided in 6 ampouls of 300 ml capacity and subjected to the sodiumalcohol treatment. The analytical composition of the resulting reaction products are shown in Table 6 together with the comparative examples using no alcohol.

As shown from the result of Table 6, effects of alcohol addition were remarkable when tert-butyl alcohol, iso-amyl alcohol, sec-octyl alcohol and lauryl alcohol were added thereto as the alcohol, whereas in the case that no alcohol was added thereto, the removal of polymerization retarding impurities (cyclopentadiene,

' acetylenes and sulfur compounds) was insufficient and no difference in effect was observed even by increasing the sodium amount.

What is claimed is:

1. A process for separating purified isoprene from a C hydrocarbon fraction derived from cracking of petroleum containing C paraffmes, C monoolefines, C diolefines, C acetylenes, a small amount of C, and C hydrocarbons and sulfur compounds which comprises:

a. removing a major portion of cyclopentadiene and acetylenes contained in said fraction as polymerization retarding impurities according to dimerization by heat soaking and distillation, and distillation, respectively and b. treating the fraction with sodium containing aliphatic monohydric alcohol having one to 18 carbon atoms in amount of not more than that of sodium by equivalent to remove the .remaining cyclopentadiene, acetylenes and sulfur compounds.

2. A process according to claim 1, wherein the amount of sodium is 0.5 1.5 parts by equivalent for 1 part by equivalent of total amount of the remaining cyclopentadiene, acetylenes and sulfur compounds.

3. A process according to claim 1, wherein the amount of sodium is 1.05 1.2 parts by equivalent for 1 part by equivalent of total amount of the remaining cyclopentadiene, acetylenes and sulfur compounds.

4. A process according to claim 1, wherein sodium is used in a state of dispersion in the inactive hydrocarbon medium.

5. A process according to claim 4, wherein sodium is dispersed in toluene, xylene, mineral oil, petrolatum, naphthalene, tetraline or heptane.

6. A process according to claim 5, wherein sodium is dispersed in toluene, xylene, mineral oil or petrolatum.

7. A process according to claim 4, wherein the sodi- TA E 5 um in the dispersion has particle sizes of 1 100 microns.

Feed stream 1 (b by weight) I c, and lighter 3.63 8. A process according to claim 1, wherein the i'z g x 'gx 5;: 40 amount of the alcohol is l 10 parts by equivalent for l 3: h i-l-b tyne 0:1 1 part by equivalent of the remaining sulfur compounds. a r 23 9. A process according to claim 1, wherein the C amount of the alcohol 1s 2 5 parts by equivalent for l Z-methyl-l-butene 8-32 part by equivalent of the remaining sulfur compounds. i sb pz ri e 1 10. A process according to claim 1, wherein the aln-pentane 251.8 5 cohol is selected from the group consisting of methyl, Z-pentene mmelhybzbmem (transcis) 218 ethyl, propyl, butyl, amyl, hexyl, octyl, nonyl, decyl, l-pentyne 0.02 lauryl, palmityl and stearyl alcohols. fy g gg g-g: 11. A process according to claim 1, wherein the al- I e a clggfllsia-peggldierllen 2g cohol 1sl selestedl from {he glrougl conlsisltinlg1 lof cyclopentenc isopropy tertuty isoamy octy an aury a co 0 s. cyclopentane 1.86 C. and heavier L69 12. A process according to claim 1 where n the C dicyclopentadierjie (120606 hydrocarbon fraction is contacted with sodium consulfur compoun s (as ppm a Tom]: [woo taming aliphatic monohydric alcohol below 40 C.

TABLE 6 Comparative Comparative Ex. 1 Ex. 2 Example 2 Example 3 Example 4 Example 6 Feed Alcohol .1 Na/imp 1.3 1.5 1.1 1.1 1.1 ROH/S compounds 5 (as S) 0 0 3. 0 3.0 3.0 Isoprene 93- 87 93. 76 94- 02 94. 00 94. 05 Cyclopentadiene (55) (10 (10 (10 Acetylenes (10 (10 (10 Sulfur compounds (24) 5) (10 (10 (10 1 Tert-butyl alcohol. 2 Iso-amyl alcohol.

3 Sec-octyl alcohol. 4 Lauryl alcohol.

5 Amount of polymerization-retarding impurities (cyclopentadiene plus acetylenes plus sulfur compounds) ratio by equivalent.

REMAnKs.Reat-.tion conditions: 30 0., 3 hours. No'rE.( p.p.m.

. 9 1 13. A process for separating purified isoprene from a in the fraction according to distillation, C hydrocarbon fraction derived from cracking of c. removing paraffins and mono-olefines' according petroleum containing C paraffines, C mono-olefines, to extractive distillation, and C diolefines, C acetylenes, small amount of C and C ea ng the fraction with sodium containing hydrocarbons and sulfur compounds, which comprises 5 aliphatic monohydric alcohol having One to 18 steps f carbon atoms in amount of not more than that of a. removing a major portion of cyclopentadiene con- Sodlum y equilalent to remove the remaining tained in the fraction according to dimerization by Y P I EII etylenes and sulfur comheat soaking and distillation, pounds. b. removing a major portion of acetylenes contained 10 w it 

2. A process according to claim 1, wherein the amount of sodium is 0.5 - 1.5 parts by equivalent for 1 part by equivalent of total amount of the remaining cyclopentadiene, acetylenes and sulfur compounds.
 3. A process according to claim 1, wherein the amount of sodium is 1.05 - 1.2 parts by equivalent for 1 part by equivalent of total amount of the remaining cyclopentadiene, acetylenes and sulfur compounds.
 4. A process according to claim 1, wherein sodium is used in a state of dispersion in the inactive hydrocarbon medium.
 5. A process according to claim 4, wherein sodium is dispersed in toluene, xylene, mineral oil, petrolatum, naphthalene, tetraline or heptane.
 6. A process according to claim 5, wherein sodium is dispersed in toluene, xylene, mineral oil or petrolatum.
 7. A process according to claim 4, wherein the sodium in the dispersion has particle sizes of 1 - 100 microns.
 8. A process according to claim 1, wherein the amount of the alcohol is 1 - 10 parts by equivalent for 1 part by equivalent of the remaining sulfur compounds.
 9. A process according to claim 1, wherein the amount of the alcohol is 2 - 5 parts by equivalent for 1 part by equivalent of the remaining sulfur compounds.
 10. A process according to claim 1, wherein the alcohol is selected from the group consisting of methyl, ethyl, propyl, butyl, amyl, hexyl, octyl, nonyl, decyl, lauryl, palmityl and stearyl alcohols.
 11. A process according to claim 1, wherein the alcohol is selected from the group consisting of isopropyl, tert-butyl, isoamyl, octyl and lauryl alcohols.
 12. A process according to claim 1, wherein the C5 hydrocarbon fraction is contacted with sodium containing aliphatic monohydric alcohol below 40*C.
 13. A process for separating purified isoprene from a C5 hydrocarbon fraction derived from cracking of petroleum containing C5 paraffines, C5 mono-olefines, C5 diolefines, C5 acetylenes, small amount of C4 and C6 hydrocarbons and sulfur compounds, which comprises steps of a. removing a major portion of cyclopentadiene contained in the fraction according to dimerization by heat soaking and distillation, b. removing a major portion of acetylenes contained in the fraction according to distillation, c. removing paraffins and mono-olefines according to extRactive distillation, and d. treating the fraction with sodium containing aliphatic monohydric alcohol having one to 18 carbon atoms in amount of not more than that of sodium by equivalent to remove the remaining cyclopentadiene, acetylenes and sulfur compounds. 